check: Simplify blob status tracking

UnusedBlobs now directly reads the list of existing blobs from the
repository index. This removes the need for the blobStatusExists flag,
which in turn allows converting the blobRefs map into a BlobSet.

internal/checker/checker_test.go

@@ -181,7 +181,7 @@ func TestUnreferencedBlobs(t *testing.T) {
 	test.OKs(t, checkPacks(chkr))
 	test.OKs(t, checkStruct(chkr))
 
-	blobs := chkr.UnusedBlobs()
+	blobs := chkr.UnusedBlobs(context.TODO())
 	sort.Sort(blobs)
 
 	test.Equals(t, unusedBlobsBySnapshot, blobs)